It has heretofore been known that a fluorescent layer comprising a mixture of a red emission fluorescent substance with a green or blue emission fluorescent substance, provides various emission colors depending on their mixing ratio. In particular, electron-beam excited display tubes such as color and monochrome cathode-ray tubes and low velocity electron excited fluorescent display tubes can display images and characters very minutely as the qualities of computer terminals or televisions are highly improved. Consequently, as a viewer has more chances of closely seeing the screen of the display tube, significant disadvantages which have previously been ignored, have come in the limelight. The typical disadvantage among them is a problem of color unevenness. Specifically, when the emission screen made up of the fluorescent mixture including the above two or three components is observed from a certain distance, the screen color looks like a single particular color. However, as the viewer approaches the fluorescent screen, the screen color looks like the color of each individual fluorescent substance, and this is visually recognized as color unevenness. In particular, since generally human vision is strongly sensitive to change in red color, this phenomenon causes human being to visually recognize red unevenness and makes the screen unsightly. Hence, there has been a strong desire to eliminate this disadvantage.
Recently, in order to lower the transmittance of a faceplate so as to relieve the eye fatigue from the viewpoint of human engineering, there has been demanded development of a display tube having a brightness superior to that of prior art. On the other hand, as rare earth elements have been increasingly used as materials in various technical fields, their prices have greatly increased, which leads to an increase in the prices of fluorescent substance containing rare earth elements. Therefore, reduction of those prices has been also demanded.
Accordingly, the object of the present invention is to provide an electron-beam excited display tube having a very high brightness and low cost fluorescent screen without any color unevenness.
The present inventors have made intensive investigations and have found out that the above object can be successfully achieved by using, as a fluorescent layer, a binary or ternary fluorescent mixture comprising (1) a red emission component composed of a europium-activated rare earth oxysulfide having a europium concentration (hereinafter referred to simply as "Eu concentration") of 0.05 to 2.0 mol %; and (2) a green or blue emission component having a particular chromaticity point. The present invention has been accomplished based on this finding.
This invention is quite unexpected and remarkable because it has heretofore been thought in this technical field that an Eu concentration of lower than 2 mol % is not suitable from the viewpoint of color and brightness according to Denki Gakkai (the Institute of Electrical Engineers of Japan), the data provided by Denshi Sochi (Electronic Device) Research Group, Data No. EDD 75-13 to 23 "Transition strength of Eu.sup.3+ from .sup.5 D.sub.0, .sup.5 D.sub.1 and the chromaticity of Y.sub.2 O.sub.2 S:Eu fluorescent material" February, 1975.